Traces of a radioactive substance have been found on two British Airways planes at Heathrow Airport, says BA.
Mr Litvinenko was a fierce critic of Russian President Vladimir Putin
The planes, plus a third in Moscow, are being tested as part of the probe into the death from radiation poisoning of ex-KGB agent Alexander Litvinenko.
BA is trying to make contact with up to 33,000 passengers who travelled on the 221 European flights affected, including the London to Moscow route.
The airline said it had been advised the risk to public health was low.
Passengers are asked to check the flight details BA is publishing on its website and to contact NHS Direct or a special helpline number if they travelled on the affected services.
BA told the BBC's Moscow bureau the third plane was currently at the city's Domodedovo airport.
A British team - thought to be police experts - will go to Moscow shortly to test the aircraft.
The airline said it had not been confirmed when the Boeing 767s could have been contaminated but forensics experts were "looking back to the end of October".
All flight numbers published on the BA website
The BBC's Richard Galpin said the traces could be there from anyone who had been in contact with Mr Litvinenko, or could have come from someone bringing the substance to the UK.
Initial results of the forensic tests had shown very low traces of a radioactive substance onboard two of the three aircraft, said BA.
Traces of radioactive polonium-210 were discovered in Mr Litvinenko's body when he died in London last week.
More traces of the substance have been found at venues he visited in the capital on 1 November.
The Health Protection Agency - which has been assessing people potentially at risk of contamination - has reassured the public that the risk of having been exposed to the substance remains low.
Chief executive Professor Pat Troop said: "What we have heard is that it's either traces or very low levels and what we have learnt so far in our investigation... is that where we have got these areas of low level radiation it doesn't seem to pose a significant health threat."
BA was contacted by the government on Tuesday night and took the three planes out of service to let forensic tests go ahead.
Contact with carrier's sweat or urine could lead to exposure
But polonium-210 must be ingested to cause damage
Radiation has very short range and cannot pass through skin
Washing eliminates traces
The airline said the investigation was confined to those three B767s, which would remain out of service until further notice.
BA chief executive Willie Walsh said: "I am advised that the health risk is actually very low. We've identified all 221 flights that have been operated by the three aircraft since 25 October and those flights are on our website.
"I would advise passengers with any concerns to check on the website first to see if they were on one of the flights involved and then contact NHS Direct or contact their local doctor."
An estimated 3,000 staff would also need to be checked, he added.
Home Secretary John Reid chaired a meeting of the government's COBRA committee on Wednesday, receiving updates from police, health authorities and others on the Litvinenko case.
A Home Office spokesman said Mr Reid expected to make a statement to Parliament concerning the investigation on Thursday.
Friends have said Mr Litvinenko was poisoned because of his criticism of Russia but the Kremlin has strongly denied any involvement.
British Airways has set up a special helpline for customers in the UK on 0845 6040171 or 0191 211 3690 for international calls.
Passengers who travelled on those flights and want further advice are advised to telephone NHS Direct on 0845 4647.